Airborne Wind Energy: Trajectory Tracking Controller Design of a Tethered Aircraft

Önen A. S. , Tekinalp O.

Wind Energy Science Conference, Hannover, Germany, 25 - 28 May 2021, vol.10, pp.119-120

  • Publication Type: Conference Paper / Summary Text
  • Volume: 10
  • City: Hannover
  • Country: Germany
  • Page Numbers: pp.119-120
  • Middle East Technical University Affiliated: Yes


Airborne wind energy has been intensely studied by the researchers, especially in the last decade

(Ahrens, Diehl and Schmehl, 2014). It stems from the crosswind flight concept that was originally

introduced by Loyd (1980). The improvements in the micro-computer and sensor technology, helped

designers use complex control algorithms and paved the path to the development of airborne wind

energy concept as an alternative to the conventional wind turbines. This new concept is

advantageous when compared to the classical wind turbines since it can be used in higher altitudes

that have a bigger wind energy potential. Moreover, due to not needing a concrete construction it is

an applicable concept in offshore as well. Although there are different types and various design

philosophies behind, using a flying object that is connected to the ground by a tether in this evolving

technology is common.

In this study, a winged airplane is used as an energy supplier. The mathematical model of the

airplane is constructed that is composed of aerodynamic, environmental, gravitational sections and

nonlinear equations of motion. (Önen, 2015) The mathematical model uses the aerodynamic control

surfaces such as ailerons, elevator and rudder. In constructing the aerodynamic model, variation of

the nondimensional force and moment coefficients according to varying angles of attack, sideslip

angle and control surface deflections are analyzed and obtained using an analysis software. The

maximum power that can be generated according to varying wind speed conditions is calculated. The

control problem of the crosswind flight is addressed in the paper which is fundamental to the control

architecture of airborne wind energy systems. There are two main approaches in controlling airborne

systems. First approach is based on online optimization of the power output of the system,

employing nonlinear model predictive control (NMPC) (Ilzhöfer, Houska and Diehl, 2007; Houska and

Diehl, 2010; Canale, Fagiano and Milanese, 2007; Canale, Fagiano and Milanese, 2008). In the second

method a trajectory tracking control methodology is preferred that separates the optimal trajectory

estimation and control problem from each other. The second methodology is used in this paper and

the trajectory tracking problem is formed as shown in Figure 1. The trajectory tracking controller is

producing the angle commands by using the difference between the actual position of the aircraft

and the desired trajectory and is mainly based on geometrical considerations. (Jehle, 2012, p.28) The

quaternion based nonlinear attitude controller is designed, producing steering commands by using

the difference between the angle command that is produced by the tracking controller and the actual

attitude of the aircraft. By expressing the current attitude and the desired attitude with quaternions,

the attitude control is realized by using the so called “to-go” quaternions. (Ariyibi and Tekinalp, p.6)

The optimal trajectory estimation that is necessary to produce the maximum amount of power is not

addressed in this study and left as a future work. The response of the controller is tested in a

simulation environment by using a pre-defined trajectory.